Chemistry and Society
Unit 46: Chemistry and the Environment
1. Define the terms pollution and pollutant.
Ans: Pollution is the introduction of waste matter or energy into the environment by humans. It causes damages or deterioration to living systems and/or the environment.
A pollutant refers to any substances or energy produced through human activities which, at an unacceptable level, causes damages to the environment or is harmful to living systems.
2. What are the major air pollutants?
Ans: The major air pollutants include carbon monoxide, sulphur dioxide, nitrogen oxides, hydrocarbons, ozone and particulates.
3. What are their sources or formation with the help of equations?
Ans: The major source of carbon monoxide is motor car exhaust. It is produced through incomplete combustion of carbon-containing fuels, such as petrol, coal and wood. For example, octane may be burnt incompletely as follows:
C8H18 (g) + 9O2 (g) → 3CO2 (g) + 3CO (g) +2C(s) +9H2O (l)
Sulphur dioxide comes mainly from burning fossil fuels (e.g. fuel oil and coal) which contain sulphur. It is also produced during the operations in power stations and metal shelters in which sulphur-containing coal is used as the fuel, and also the manufacturing industries in which fuel oil is burnt.
S(s) + O2 (g) → SO2 (g)
Nitrogen oxides are produced whenever fuels are burnt at a very high temperature. It also comes from diesel-driven vehicles running in the territory. Diesel engines produce much less carbon monoxide but a lot more nitrogen oxides than petrol engines.
N2(g) + O2(g) → 2NO(g)
2NO (g) + O2 (g) → 2NO2 (g)
Hydrocarbons are formed from the evaporation of organic materials such as petrol, diesel and solvents. They are also found in car exhaust as unburnt hydrocarbons.
Ozone is produced by the reaction of oxygen gas with free atoms of oxygen which are formed from the reactions between nitrogen oxides and hydrocarbons in sunlight.
O2 (g) + O O3 (g)
Oxygen
Radical
The particulates are produced during the operations in incinerators, factories, diesel vehicles, construction sites and coal or charcoal burners. They are solid or liquid particles and are so small that they remain suspend in air for a long period of time.
4. What are the common effects of these pollutants?
Ans: Carbon monoxide reduces the amount of oxygen delivered to body organs and tissues or even causes death.
Sulphur dioxide will impair the respiratory function and aggravate existing respiratory diseases (e.g. bronchitis and asthma) and cardiovascular diseases.
Nitrogen oxides will irritate the lungs and lower the resistance to respiratory infections, and causes the formation of photochemical smog and acid rain.
Hydrocarbons will cause photochemical smog. Some of them are carcinogenic.
Ozone will affect the respiratory system and increase susceptibility to respiratory infections, and damage plants.
The particulates will impair the lung function by hindering gas exchange and damaging lung tissues, and cause respiratory illnesses.
5. What are the factors that the harmful effects of pollutants depend on?
Ans: The harmful effects of pollutants depend on their concentrations and the duration of exposure to them.
6. What does ppm stand for?
Ans: ppm (parts per million) is the number of molecules of pollutants per million (10 ) molecules in air. It is thus a ratio with no unit. Based on the Avogadro’s law, ppm is also equal to the number of volumes of pollutants per million volumes of air.
7. Express 0.03 ppm SO2 as μg m ³.
Ans: In 1m³ of air, there is 0.03\10 m³ carbon monoxide.
Using the ideal gas equation,
PV=nRT
= (Mass of SO2 / M) RT
where M is the molar mass of SO2.
Mass of SO2 = PVM
RT
= (101X10³Pa) (0.03/10 m³)(64 g mol ¹)
( 8.314J K ¹ mol ¹)(298 K)
= 78.3 μg
Therefore, the concentration of sulphur dioxide is 78.3μg m ³.
8. Decide whether the unpolluted rainwater is acidic or alkaline? Explain with the help of equations?
Ans: The unpolluted rainwater is slightly acidic, with a pH value of about 5.7. It is because as it falls, the rainwater reacts with carbon dioxide in the air to form carbonic acid.
CO2 (g) + H2O (g) → H2CO3 (aq)
9. What are the two major air pollutants responsible for the formation of acid rain? Write down the relevant equations.
Ans: Acidic gaseous pollutants like sulphur dioxide and nitrogen dioxide are responsible for the formation of acid rain. Sulphur dioxide is mainly released from the burning of fossil fuels in power stations.
2SO2 (g) + O2 (g) → 2SO3 (g)
SO3 (g) +H2O (l) → H2SO4 (aq)
2NO (g) + O2 (g) → 2H2SO4 (aq)
4NO2 (g) + 2H2O (l) +O2 (g) → 4HNO3 (aq)
10. What are the effects of the acid rain on the environment?
Ans: Acid rain causes a lot of harmful effects on environment. In the water of pH lower than 4.5, calcium metabolism in freshwater fish will be affected, leading to poor health and stunted growth. As a result, the diversity and population of some freshwater species will be reduced. In soil of pH lower than 4.5, absorption of essential nutrients by plants will be affected, resulting in the death of plants.
Acid rain also corrodes metals and accelerates the rate of deterioration of buildings, rocks and statues.
11. What does it mean by photochemical smog?
Ans: The word “smog” comes from two words: smoke and fog. Photochemical smog is a mixture of pollutants including particulates, nitrogen oxides, ozone, aldehydes, peroxyacetyl nitrate (PAN) and unreacted hydrocarbons, etc. A brownish haze that irritates our eyes is an indicator of photochemical smog. Nitrogen dioxide is responsible for the brownish color of the haze.
12. How is the photochemical smog formed?
Ans: Reaction that lead to the formation of photochemical smog are initiated by sunlight and involve hydrocarbons and nitrogen oxides emitted from automobiles. The combination of sunlight, particulate matters and abundant pollutants present in modern cities provide favourable conditions for smog formation.
Nitrogen dioxide from automobile exhaust first absorbs sunlight and breaks down into nitrogen monoxide and free oxygen atom which is very reactive.
UV light
NO2 (g) → NO (g) + O (g)
The oxygen atom then reacts with other components of automobile exhaust (e.g.unburnt hydrocarbons) and those of the atmosphere (e.g. oxygen and water vapour) in a series of complex reactions to produce a variety of lachrymatory and toxic chemicals such as peroxyacetyl nitrate (PAN).
Very much simplified, some of the principal reactions involved in the formation of photochemical smog are illustrated below:
O (g) + O (g) → O3 (g)
O (g) + hydrocarbon → aldehydes
O3 (g) + hydrocarbons → aldehydes
Hydrocarbons + O2 (g) + NO2 (g) → lachrymatory substances,
including PAN
13. What are the effects of the photochemical smog?
Ans: Photochemical smog causes headaches, eye, nose and throat irritation, worsening of the lung function, coughing and wheezing. Ozone attacks the C=C linkage in rubber in a process known as ozonolysis. It can also cause rubbers and fabrics to deteriorate and car tyres to crack.
14. What is the formula of ozone?
Ans: Ozone is formed from atmosphere oxygen by absorption of ultra-violet radiation of the right energy (wavelength 250nm), which dissociates oxygen molecules into free oxygen atoms.
O2 (g) → 2O (g)
The oxygen atoms produced react with other oxygen molecules to form ozone.
O (g) + O2 (g) → O3 (g)
15. What is the formation of the ozone in the lower part of atmosphere?
Ans: In the lower part of the atmosphere, (i.e. the troposphere), ozone can be formed in a number of ways: by the reaction between nitrogen oxides and hydrocarbons in sunlight; by electric sparks which occur in car engines and electrical appliances (e.g. photocopiers and electric motors ); and by lightning. It can also be formed by passing a steam of oxygen through an electric discharge. However, ozone is very reactive (oxidizing). So it does not stay in the atmosphere for a long time.
16. What is the formation of the ozone in the stratosphere?
In the stratosphere, ozone is formed from atmospheric oxygen by absorption of ultraviolet radiation of the right energy (wavelength 250nm), which dissociates oxygen molecules into free oxygen atoms. The oxygen atoms produced react with other oxygen molecules to form ozone.
O2(g) à 2O(g) O(g) + O2(g) à O3(g)
17. What happen to ozone if it absorbs UV radiation of wavelength 215 to 295nm?
When ozone absorbs ultraviolet radiation of wavelength 215 to 295, it undergoes photodissociation and splits up again.
O3(g) àO2(g) + O(g)
18. What is the significance of the above reactions?
The reactions are responsible for the vital screening effect of ozone and screen off the harmful ultraviolet radiation. As ozone is constantly created and destroyed in the stratosphere, its concentration remains constant.
19. What are the effects of ozone to human?
Ozone filters out 99% of the dangerous ultraviolet radiation from the sun. When the ozone layer is depleted, increased penetration of solar ultraviolet radiation is likely to have high impact on human health with potential risks of eye diseases, skin cancer and infectious diseases.
20. What would happen if the ozone layer becomes thinner?
If the ozone layer becomes thinner, it will affect the lives of living organisms on the Earth. There is an increase in the amount of ultraviolet light reaching the Earth’s surface as its function is to shield us from the ultraviolet radiation of the sun. This will lead to an increase in the incidence of skin cancer, genetic mutation, cataract, and a decrease in the yield of crops.
21. What substances would cause a decrease of ozone in the atmosphere?
Ozone is highly reactive and easily broken down by man-made chlorine and bromine compounds. Chlorofluorocarbons (CFCs) which are compounds containing chlorine, fluorine and carbon and other ozone-depleting substances ODS would cause a decrease of ozone in the atmosphere. Strong UV light breaks apart the ODS molecules. CFCS, HCFCS, carbon tetrachloride, methyl chloroform release chlorine atoms, and halons and methyl bromide release bromine atoms. The chlorine and bromine atoms destroy the ozone.
22. Using one of the substances in Q.21, write down the relevant equations for depletion of the ozone in the atmosphere.
One of the substances is CCl3F. After absorbing ultraviolet radiation, the equation is:
CCl3F à CCl2F (g) + Cl (g)
Then, the free chlorine radicals (Cl ) steps readily react with ozone. Here are the equations:
Cl (g) + O3(g) à ClO (g) + O2(g)
ClO (g) + O3 à Cl (g) + 2O2(g)
23. Give some alternatives for CFCs compounds.
CFCs such as CCl3F and CCl2F2 are used as aerosol propellants because they are good solvents with low boiling points and are odourless, non-flammable, stable and non-toxic. For CCl2FCClF2, it can dissolve grease and it is used as solvents in cleaning electronic components and metals. A series of CFCs, Freon (CCl2F2), absorbs heat of vaporization on evaporation and cools the surroundings. So, it is widely used as refrigerants in refrigerators and air conditioning units. A volatile CFC such as CCl3F is incorporated into the plastics in order to make foam plastics. The heat evolved during the polymerization reaction vaporizes the CFC which forms tiny bubbles in the plastics.
24. What are the causes of water pollution?
Water pollution is caused by livestock waste, oil spillages, residues of pesticides, detergents in sewage and industrial effluents.
25. What are the effects of water pollution?
When there is an excessive amount of organic matter discharged into water, the existing micro-organisms will grow rapidly and oxygen is also depleted rapidly. Thus, anaerobic micro-organisms break down the organic matter, releasing noxious gases and leading to the death of aquatic organisms.
For oil spillages, oil clogs the feathers of sea birds and prevent them from flying or swimming. It also interferes with the insulation of the feathers of birds and causes the death of them from cold or pneumonia. Oil will poison the birds by ingestion and prevent dissolution of oxygen, causing the death of aquatic organisms. Besides, oil deposited on beaches spoils recreational resorts.
When the residues of pesticides entry into the water, they will accumulate along the food chain and have the potential for damaging internal organs. Also, the reproduction of the animals will be affected and even exerted harmful effects.
Discharge of detergents in sewage causes eutrophication brought about by phosphates found in some detergents. It also causes foaming in rivers and lakes that leads to the death of aquatic organisms.
Industrial effluents may contain heavy metals which are toxic. They will accumulate along the food chain and cause death of animals when a toxic level is reached.
26. What does DO stand for?
DO stands for dissolved oxygen.
27. What is the indication of the DO value?
DO value indicates the oxygen content in water. It is measured in mg of oxygen per dm of water or percentage saturation of dissolved oxygen.
28. What does BOD stand for?
BOD stands for biochemical oxygen demand.
29. What does it mean by five-day BOD?
BOD is determined in the laboratory by incubating a water sample for five days at
20 C and measuring the amount of oxygen consumed and it is known as five-day BOD.